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In a blind fish that dwells in deep, dark Mexican caves, scientists have found evidence for a long-debated mechanism of evolutionary change that is distinct from natural selection of spontaneously arising mutations, as reported this week in the journal Science.
The eyeless cavefish Astyanas mexicanus is "a special system in which we can look at evolution in action," says article co-author William Jeffery, a senior adjunct scientist at the Marine Biological Laboratory (MBL) in Woods Hole, Mass., and a professor at the University of Maryland. The Science study was led by Nicolas Rohner and Clifford J. Tabin at Harvard Medical School's Department of Genetics.
Using the cavefish, the team demonstrated for the first time in nature how "standing" or "cryptic" genetic variations in an animal, which have been inherited from prior generations without causing any physical changes in the animal, can be "unmasked" by the shock of entering a new environment. Gene variants that improve the animal's ability to adapt to that new environment can then be selected for, and passed on to its progeny. This is distinct from the established evolutionary mechanism of "de novo" genetic mutations that arise by chance after the animal has entered the new environment, which also provide a substrate upon which natural selection can act.
"That is controversial," Jeffery says, noting that it is difficult even for scientists in the field to conceive of the purpose of losing eyes. The paper proposes several possible advantages of eyelessness, such as allowing the animal to conserve the large amount of energy it takes to maintain an eye and to expend it, instead, on traits that are useful in a dark environment.
The descent of the surface-dwelling Astyanas mexicanus into the cave a few million years ago "is a very, very recent event, in evolutionary terms," Jeffery says. "We are talking about a rapid evolutionary process here, as opposed to the 500 million years of natural selection that have unfolded since most animal [groups] appeared during the Cambrian Period. The fact that these eyeless cavefish are so young makes them very attractive to understand evolutionary processes at their beginning."
Rohner H, Jarosz DF, Kowalko JE, Yoshizawa M, Jeffery WR, Borowsky RL, Lindquist S, and Tabin CJ (2013) Cryptic variation in morphological evolution: HSP90 as a capacitor for loss of eyes in a cavefish. Science: 10.1126/science.1240276
Article adapted by Medical News Today from original press release. Click 'references' tab above for source.
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